There is a critical need for safe and effective antiretroviral treatment (ART) regimens for HIV-2 infection. This is especially true in West Africa where the vast majority of the 1-2 million individuals infected with HIV-2 live and were access to effective ART for HIV-2 is limited. HIV-2 is intrinsically resistant to many of the standard antiretrovirals used to treat HIV-1; including the non-nucleoside reverse transcriptase inhibitors (NNRTI) and the fusion inhibitor enfuvirtide (T-20). In addition, mutations conferring broad resistance to nucleoside/nucleotide reverse transcriptase inhibitors (NRTI) are frequently observed in HIV-2 from patients receiving ART. Although antiretroviral protease inhibitors (PI) can be used effectively to treat HIV-2, HIV-1 and HIV-2 also exhibit important differences in their susceptibilities with studies indicating that saquinavir (SQV), lopinavir (LPV), and darunavir (DRV) are the only potent PI's against HIV-2 replication and cross-resistance is frequent. Unfortunately, the utility of CCR5 co-receptor antagonists (maraviroc) is hampered by HIV-2's ability to use multiple co-receptors for cell entry. An increasing body of evidence supports the potential utility of integrase inhibitors (INI) against HIV-2, however there have been no clinical trials to assess their effectiveness. These limitations present major challenges to HIV-2 treatment, particularly in the areas in which it is most prevalent. Current WHO guidelines and National Programs in West Africa recommend for initial, 1st-line ART for HIV-2 infection: 2 NRTI (typically AZT+3TC) + lopinavir/ritonavir (LPV/r). However clinical and virologic failure rates are high and development of multiclass resistance is common. Complicating assessment of HIV-2 patients failing ART, there is no routine HIV-2 viral load or drug resistance testing available in most of West Africa. In addition, there are no WHO recommended, proven or effective 2nd-line ART regimens to treat HIV-2 infected individuals failing 2 NRTI + LPV/r. Recently however, in Senegal, the INI, raltegravir and the 2nd- generation PI, darunavir, recently have become available for 2nd-line ART in HIV-2 infection through the Initiative Sngalaise d'Accs aux Antirtroviraux (ISAARV) and algorithms for their use in HIV-2 infection are being developed by ISAARV. In order to address these critical challenges and to inform public health based approached to treatment and care of HIV-2 infected individuals in West Africa, we will undertake the following aims in our proposed grant. AIM 1: Develop, implement and evaluate outcomes of a new HIV-2 viral load and ARV resistance-informed algorithm for 2nd-line ART in HIV-2 infected patients in the Initiative Sngalaise d'Accs aux Antirtroviraux (ISAARV) program. AIM 2: Determination of genotypic and phenotypic susceptibility, resistance mechanisms and pathways, of HIV-2 to novel and pipeline antiretroviral agents. AIM 3: Development and validation for clinical diagnostic use of a novel HIV-2 and HIV-1 total nucleic acid detection assay. Our longstanding, multidisciplinary effort to develop evidence-based treatment and care for HIV-2 infected adults in Senegal has high potential to significantly improve patient outcomes.